Magnetic Particle Imaging (MPI) is an emerging medical imaging modality. The first versions of MPI were two-dimensional in that they produced two-dimensional images. Newer versions are three-dimensional (3D). A four-dimensional image of a non-static object can be created by combining a temporal sequence of 3D images to a movie, provided the object does not significantly change during the data acquisition for a single 3D image.
MPI is a reconstructive imaging method, like Computed Tomography (CT) or Magnetic Resonance Imaging (MRI). Accordingly, an MP image of an object's volume of interest is generated in two steps. The first step, referred to as data acquisition, is performed using an MPI scanner. The MPI scanner is arranged to generate a static magnetic gradient field, called the “selection field”, which has a (single or more) field-free point(s) (FFP(s)) or a field-free line (FFL) at the isocenter of the scanner. Moreover, this FFP (or the FFL; mentioning “FFP” in the following shall generally be understood as meaning FFP or FFL) is surrounded by a first sub-zone with a low magnetic field strength, which is in turn surrounded by a second sub-zone with a higher magnetic field strength. In addition, the scanner has means to generate a time-dependent, spatially nearly homogeneous magnetic field. Actually, this field is obtained by superposing a rapidly changing field with a small amplitude, called the “drive field”, and a slowly varying field with a large amplitude, called the “focus field”. By adding the time-dependent drive and focus fields to the static selection field, the FFP may be moved along a predetermined FFP trajectory throughout a “volume of scanning” surrounding the isocenter. The scanner also has an arrangement of one or more, e.g. three, receive coils and can record any voltages induced in these coils. For the data acquisition, the object to be imaged is placed in the scanner such that the object's volume of interest is enclosed by the scanner's field of view, which is a subset of the volume of scanning.
The object must contain magnetic nanoparticles or other magnetic non-linear materials; if the object is an animal or a patient, a tracer containing such particles is administered to the animal or patient prior to the scan. During the data acquisition, the MPI scanner moves the FFP along a deliberately chosen trajectory that traces out/covers the volume of scanning, or at least the field of view. The magnetic nanoparticles within the object experience a changing magnetic field and respond by changing their magnetization. The changing magnetization of the nanoparticles induces a time-dependent voltage in each of the receive coils. This voltage is sampled in a receiver associated with the receive coil. The samples output by the receivers are recorded and constitute the acquired data. The parameters that control the details of the data acquisition make up the “scan protocol”.
In the second step of the image generation, referred to as image reconstruction, the image is computed, or reconstructed, from the data acquired in the first step. The image is a discrete 3D array of data that represents a sampled approximation to the position-dependent concentration of the magnetic nanoparticles in the field of view. The reconstruction is generally performed by a computer, which executes a suitable computer program. Computer and computer program realize a reconstruction algorithm. The reconstruction algorithm is based on a mathematical model of the data acquisition. As with all reconstructive imaging methods, this model can be formulated as an integral operator that acts on the acquired data; the reconstruction algorithm tries to undo, to the extent possible, the action of the model.
Such an MPI apparatus and method have the advantage that they can be used to examine arbitrary examination objects—e.g. human bodies—in a non-destructive manner and with a high spatial resolution, both close to the surface and remote from the surface of the examination object. Such an apparatus and method are generally known and have been first described in DE 101 51 778 A1 and in Gleich, B. and Weizenecker, J. (2005), “Tomographic imaging using the nonlinear response of magnetic particles” in Nature, vol. 435, pp. 1214-1217, in which also the reconstruction principle is generally described. The apparatus and method for magnetic particle imaging (MPI) described in that publication take advantage of the non-linear magnetization curve of small magnetic particles.
In contrast to established imaging modalities like MRI and CT, no simple mathematical transform has yet been identified for MPI to reconstruct images from the acquired data. Therefore, MPI image reconstruction requires knowledge of a “system function” describing the system response to a given spatial distribution of particles, i.e., mapping particle position to frequency response. To solve the reconstruction problem, the system function has to be inverted, usually requiring some regularization scheme.
The system function can be determined experimentally by measuring the magnetization response of a point-like sample at a large number of spatial positions corresponding to the number of image pixels or voxels. This calibration procedure requires quite long acquisition times, in particular in order to obtain a reasonable signal-to-noise ratio (SNR). More details with respect to the acquisition, features and use of the system function as well as preferred embodiment for faster acquisition and for reducing the storage space for storing the system function can be found in WO 2010/067248 A1 and WO 2010/067264 A1, which details are herein incorporated by reference.
Large spatial coverage in MPI can be achieved by moving the field of view (FOV) encoded by magnetic drive fields using additional homogeneous offset fields called magnetic focus fields. To date, magnetic focus fields have been applied e.g. to generate a slow continuous FOV motion with 3D encoding as described in J. Rahmer et al., “Continuous Focus Field Variation for Extending the Imaging Range in 3D MPI”, Magnetic Particle Imaging: A Novel SPIO Nanoparticle Imaging Technique 140 (2012): 255. For a continuous FOV motion during imaging, “slow” means that the shift during one encoding period remains below the reconstructed spatial resolution. In the above mentioned paper of J. Rahmer, an uncompromised image quality was demonstrated for a shift velocity of about 20 mm/s, which is sufficiently far below the 50 mm/s that correspond to the ratio between the 3D encoding time of 21.5 ms and the resolution of roughly 1 mm.